A complex power system includes various kinds of devices, and a line per se may also be regarded as a device. A variety of factors may cause device outage, the most typical of which is device failure. Device outage affects power supply of power unit. Device failure-incurred device outage is unpredictable, which is adverse to taking redemptive measures to the effect of outage. One way to reduce this unpredictable outage is to maintain the device. A device with proper maintenance has a longer service life and a lower failure rate than one that is ill maintained or not maintained at all. Though device maintenance per se will also cause device outage, such outage is predictable and helps take redemptive measures in advance to the effect of outage. Due to features of power system, outage of a device may trigger chain outage of more other devices. For example, if the outage device is a sole power source device of another device, then the another device will be in outage accordingly. For another example, since a live device cannot be maintained, when maintaining a device, besides the device per se which needs to be in outage, a respective power source device for the device is also required to be in outage.
In order to reduce the impact of device outage to the power unit, a direct method is to set backup. For example, an important power unit, for example a medical apparatus or a scientific research instrument, always has two sets of power supply systems, where when one set of system cannot continue power supply due to device outage, the other set of system takes the responsibility for power supply. However, due to the huge amount of devices included in a power system, it is impossible to set backup for each device.